1. Field of the Invention
The invention relates to a method for producing a large-volume, cup-shaped or tub-shaped container of thermoplastic material, wherein a hose-shaped pre-form of compact plastic material is extruded to a predetermined length, is widened by means of spreading elements to a predetermined size, and subsequently formed by forming air to the container, as well as to a device for performing the method.
2. Description of the Related Art
Transport and storage containers of compact plastic material, which are cup-shaped or tub-shaped, i.e., open at the top, are known which are initially produced by an extrusion method, in which a hose-shaped pre-form is produced, and a subsequent blowing method in a hollow, at least two-part mold, in which the hose-shaped pre-form is widened by blowing air and brought into contact with the inner wall of the hollow mold. The thus shaped container is then still closed at the top. By cutting off the upper cover area, the required container open at the top is then produced. This cutting step represents an additional working step which requires a special cutting device. Moreover, the upper edge of the container has a relatively minimal strength which is disadvantageous when the container is used as a protective container for another container.
A protective container for a large-volume container is known by which the aforementioned disadvantages are avoided.
This protective container has, however, a wall which is comprised of three layers wherein the inner layer is formed of a foamable plastic material. Such a wall, on the one hand, entails a high material consumption and requires, on the other hand, an extrusion device with relatively high capital investment. In addition to the extrusion device a mold is required which is complex and comprised of an inner mold and an outer mold and must withstand the foaming pressure of the inner layer. The method, as already mentioned above, can be performed only with a foamable inner layer. From a U.S. Pat. No. 5,474,734, a method for manufacturing hollow mold parts of thermoplastic material by a blowing process is known in which the finished mold parts are to have different wall thicknesses. In order to achieve this, two spreading elements, which can be supplied with a cooling liquid, are introduced from below into an extruded, hose-shaped pre-form with constant wall thickness and are moved apart. This reduces the wall thickness of the hose-shaped pre-form in the two areas which are not resting against the two spreading elements while the wall thickness remains constant in the area where the hose-shaped pre-form rests against the cooled spreading element. As soon as the desired degree of spreading has been reached, the two spreading elements are removed from the pre-form. Subsequently, a two-part blowing mold is closed about the pre-form and its lower and upper ends are squeezed together. When closing the mold, a blowing spike is introduced into the pre-form by means of which the pre-form is then blown to the finished, closed molded part.
The invention has thus the object to provide a method for producing a large-volume, cup-shaped or tub-shaped container of thermoplastic material which can be performed with an extremely simple extrusion device and molds.
As a solution to this object it is suggested according to the invention for the method of the aforementioned kind that a core is moved into the extruded, hose-shaped and widened pre-form from below and the lower free end of the widened pre-form is clamped sealingly against the core.
Such a method does not require a complicated and complex extrusion device for producing a large-volume, cup-shaped or tub-shaped container that is open at the top and has a wall of compact plastic material. The part of the mold which determines the final shape of the container can be of a simple design wherein in many cases a core as a shaping body is already sufficient. A special mold of at least two parts is then no longer needed. When manufacturing the container on a core, blowing air is exclusively used as the forming air. In the process of manufacturing in a mold, the forming air can be blowing air as well as a vacuum.
The invention will be explained in the following in more detail with the aid of embodiments illustrated in the drawing.